Broadly speaking, this invention relates to altitude sensing means suitable for use with altitude control systems or with pilot warning systems for aircraft. More particularly, in a preferred embodiment, this invention relates to a barometric altimeter combined with a radar altimeter.
A common type of altimeter is the pressure responsive altimeter which includes an aneroid barometer arrangement having expansible bellow means. Other types of altimeters have been developed based upon the principle of radar which utilizes reflected signals from the surface of the earth. These radar altimeters have means for sensing and measuring the absolute altitude above the earth's surface.
A barometric altimeter measures the pressure of atmospheric air which can be used for determining the true altitude above sea level but cannot detect actual height above terrain. In contrast, a radar sensing device accurately measures the actual height above the terrain but cannot provide reliable readings during some flight attitude conditions. For example, it is well recognized that sharp banks, dives and climbing attitudes of an aircraft will cause unreliable altimeter signals from a radar unit due to the aircraft's pitch and roll angles being greater than the radar altimeter's cone of operation. Because of this, radar altimeters generally include an indicator of whether the reading is valid or invalid. Furthermore, a failure in radar signals will cause a radical change in an altitude control system, especially during a low altitude, terrain following flight.
It has been proposed that barometric and radar altitude sensors be combined so that the advantages of both can be utilized while the disadvantages of the individual sensors are neutralized. For example, U.S. Pat. No. 3,140,483 combines barometric and radar altimeters using complex analog devices. While this patent is good for its purpose of generating an altitude error signal for driving an aircraft control surface, it does not provide a true and absolute reading of altitude above ground.